Interferometer ghosts are of concern for a number of reasons. First, pairs of reflections form low finesse etalons and imprint a rapidly oscillating transmission pattern on scene energy. Typical infra-red interferometer components give rise to an etalon fringe spacing that is comparable to the required spectral resolution of sensors used to detect the interference pattern. FIG. 1, for example, shows the transmission function for an etalon made from 10 mm of ZnSe at normal incidence. The surface reflectances are 50% and 3%. The period of the oscillation is 0.21 cm−1. A 5 mm plain parallel air gap would have an etalon period of 1.0 cm−1.
FIG. 2 shows further details of common ghost paths and their normalized intensities. In this figure, R1 and R2 are surface reflectances, P1 and P2 are primary image paths, and T1 and Rext are common ghost paths.
Very small changes in temperature may cause significant changes in the internal optical path lengths that create the etalons. If changes in optical path length occur between calibrations, the transmission patterns of the etalons shift and result in radiometric errors. A compensated Michelson interferometer, for example, has 4 potentially parallel surfaces (2 surfaces of the beamsplitter and 2 surfaces of the compensator) resulting in 6 pairs that may form etalons. Keeping the total radiometric error well below 1% requires keeping the temperature of the interferometer components constant to a fraction of 1° K.
A second problem occurs when the interferometer plates are wedged and tilted to destroy the constructive interference that produces an etalon. When none of the 4 surfaces of a compensated Michelson interferometer are mutually parallel, internal reflections deviate the ghost image away from the primary image. When such an interferometer views a scene, a second, fainter and displaced image of the scene is superimposed on the first image. Low radiance features in the scene may become contaminated by the overlying ghost image of a high radiance feature at some distance from the point of interest. This effect cannot be calibrated away. Because a blackbody has a uniform temperature and, therefore, is without any structure to view, a blackbody ghost is indistinguishable from the primary image. Furthermore, the presence of a prominent, in-focus ghost, at a distance from the primary image, may affect detector optics ensquared energy (DOEE) performance.
FIG. 3 shows output angles of ghost ray paths when two surfaces are not parallel. The angular offsets of the ghosts from the primary image are A3-A1 for the transmitted path and A2-A for the reflected path.
A third problem related to ghost images concerns the IR background energy levels. It is common to cool portions of IR radiometers in order to reduce the self emission of the sensor and drive photon shot noise down. This only works when the IR background originates within the cooled portion of the sensor. In an interferometer, on the other hand, the ghost reflections do not terminate on an interferometer housing. Running the rays “backwards” to determine what surface the detector “sees” reveals that the ghost paths do not permit the detector to see the instrument at all. The rays are found to originate within the scene itself. So cooling the interferometer only reduces emission from the mirrors and coated substrates and this turns out to be only 10-30% of the ghosted scene emissions depending upon the wavelength in question. The interferometer components' self emission is greater in the long wavelength (LW) range, where substrate and coating absorption becomes significant. These scene ghosts are the dominant source of unwanted background flux in a cooled Fourier transform spectrometer (FTS) based sensor.
Strong ghost images can result whenever two planar optical surfaces are in close proximity. Therefore, optical windows and beamsplitters commonly produce ghost images. For a predetermined application of a specific interferometer there may be a maximum acceptable magnitude of ghost images. If the actual magnitude of a ghost image is found to exceed this maximum, the ghost magnitude must be reduced. Strategies for reducing ghost magnitude include the use of high efficiency anti-reflection (AR) coatings where possible, wedging components to eliminate favorable etalon producing interference conditions, and judicious choice of wedge and tilt angles so the ghost image is driven into a wall.
In the case of interferometer ghosts, however, there are some complicating factors. First, the beamsplitting surface is, by necessity, a nearly 50% reflector. Also, AR coatings become less efficient as the spectral range increases and as the angle of incidence increases. One type of interferometer has a 45 degree incident angle and can operate from about 4 to about 15 microns. With an average AR reflectance of 4%, the two ghost ratios are 0.02 and 0.0868 for the transmitted and reflected paths, respectively.
The strong reflected path ghost is a serious concern, as it may be a source of radiometric error and/or a violation of ensquared energy performance requirements. Driving it lower can only be achieved with a better AR coating. Reducing the reflectance of the coating may be achieved in two ways. First, reducing the incidence angle from, for example, 45 to 30 degrees, probably enables reflectances near 3%. Secondly, reducing the spectral range over which the coating operates enables further reflectance reduction, assuming that it satisfies the intended use of the interferometer. For example, one use allows the operating spectral range to be reduced from about 4 to about 9 microns. With this reduction, coupled with a lower incident angle, reflectances in the 1.6-2% may be achievable. Using a two-interferometer design, in which two bands are separated (e.g. MW from about 6 to about 9 microns and SW from about 4.2 to about 4.7 microns) may make about 1% reflectance within the realm of possibility. Reducing the reflectance may reduce the magnitude of the etalon peaks, reduce the radiometric errors and reduce the IR background.
Wedging the interferometer plates may greatly reduce the peak-to-valley (P-V) variation of the etalon transmission pattern. An IR ghost still exists, but the constructive interference from multiple reflections that gives rise to the etalon is reduced to arbitrarily small values. One interferometer (referred to herein as A) incorporates a 100 microradian wedge in its beamsplitter and compensator. Another interferometer (referred to herein as B) incorporates a 1.2 milliradian wedge, and a possible air wedge of 3.8 milliradians. The difference between the two interferometers is due to the much larger field of view (FOV) of each sensor (0.963 degree) used in interferometer A. Considerable self apodization occurs and contributes nearly a ten times reduction in the etalon peak-to-valley variation. Thus, interferometer A only needs a little help to drive the etalon into the noise. The FOV of interferometer B is 0.0448 degree (782 microradians). Very little self apodization is the result in interferometer B, so a much larger wedge is required to defeat the etalon.
While these angles may reduce the etalon pattern, they may cause other undesirable effects. The wedges are great enough that their chromatic aberration may require compensation elsewhere in the optical system with a window wedged in the opposite direction. Additionally, the interferometer components in interferometer B introduce approximately 1.4% anamorphic magnification. The counter wedge may partially reduce the aberration depending upon its location and tilt.
The wedges in interferometer B may also produce ghost images that are 0.4492 and 0.4476 degrees (approximately 7.8 milliradians) from the transmitted and reflected primary image path, respectively. The air wedge causes a ghost that is 2×3.8=7.6 milliradians away. If used in certain sensors, this corresponds to a 10 FOV separation of the primary image from the ghost image at the focal planar array (FPA). There may be other weaker second generation ghosts at roughly twice this spacing, and ghosts caused by reflections between non-adjacent surfaces at larger separations.
Interferometer A does not need to be concerned about these ghost images, because the images appear approximately 0.03 degree offset from the primary image—only 3% of a FOV width away. The only effect of the ghost may be to broaden the FOV response tail very slightly. Chromatic aberration and anamorphism are <10% of that of interferometer B.
Many of the ghost images produced from interferometer B, though stronger and further offset from those produced by interferometer A, are basically DC terms. The optical path lengths are considerably different than the primary imaging optical path length. However, a full analysis of the optical paths of the most prominent ghosts has not been performed. There are several ghost paths that have equal optical path “mates” in the other arm of the interferometer. These paths, though generally longer than the primary path, interfere with each other at zero path difference (ZPD) and add to the primary AC signal.